1. Field of the Invention
The present invention relates to the field of monitoring devices using the effect of atomic absorption for quantifying particles of interest, and in particular to an atomic absorption monitor using a frequency-modulated light beam generated by a laser for ascertaining the presence and movement-related parameters of such particles.
2. Description of Prior Art
Monitors for quantifying particles involved in processes such as thin film deposition, evaporation, sputtering, laser ablation, ion milling, and secondary ion mass spectroscopy are crucial to many areas of technology. Particularly strict demands are placed on such monitors in supervising thin film deposition processes involving high temperatures, pressures, and throughputs. Ion milling and secondary ion mass spectroscopy pose no lesser monitoring challenges, especially with regard to sensitivity. To satisfy these stringent requirements, the devices need to be robust, versatile, accurate, sensitive, and they should require little or no maintenance.
In addition to detecting the quantity of the particles of interest, it is also important to determine other parameters related to their movement. For example, it is highly desirable to know how fast and in which direction a particle is moving. Such information helps to determine deposition rates, velocity profiles inside process chambers, sticking coefficients, and many other technical parameters.
Traditional rate monitors include quartz crystal monitors (QCM), quadrupole mass spectrometers (QMS), ion gauges, and electron impact emission spectrometers (EIES). Unfortunately, these monitors do not always have the requisite versatility or robustness for many applications. The monitors relying on a hot filament, including the QMS, ion gauge, and the EIES cannot be used in high-pressure applications such as sputtering. Also, when sputtering or other deposition processes are used to achieve high throughput, as is frequently done in the semiconductor industry, none of the above techniques, including the QCM technique, can be employed. The rapid coating of monitors under these conditions results in frequent downtime inside the vacuum system for cleaning operations. Such interruptions are not economically feasible. In fact, for these two reasons, rate monitors are often non-existent today in commercial semiconductor sputtering applications.
Atomic absorption monitors offer considerable advantages over the traditional devices listed above. They do not need to be placed inside vacuum chambers and thus avoid being coated by the deposition material. This contributes to their overall low maintenance requirements. Atomic absorption devices are also highly element-specific, insensitive to extreme conditions such as high temperatures and pressures, and capable of tracking low and high deposition rates.
Many existing atomic absorption monitors use a hollow cathode lamp to emit a signal beam of a characteristic wavelength. This beam passes through a chamber, e.g., a deposition chamber, containing the particles of interest. The particles absorb a portion of the signal beam which depends on their density, absorption cross section, and the length of the region containing the said particles of interest. Upon emerging from the chamber the beam is intercepted by appropriate photo-electronic detection means (e.g., a photodiode) which converts it into an electrical signal. The intensity reduction due to absorption is computed from this detected signal. Deposition rates, material component ratios, and other parameters are then derived from this result in a straightforward way.
Unfortunately, such atomic absorption monitors experience problems due to the intrinsic instability and incoherence of the light source and detection means. Both have a tendency to drift with time. In applications involving slow deposition rates, deviations of 10% per hour of the evaporation signal are not uncommon. Moreover, random noise of the detection means adds instantaneous fluctuations to the drift. Under these conditions it is next to impossible to determine additional parameters related to the movement of the particles with any accuracy. Moreover, it is also difficult to use the rapidly spreading spatially incoherent beam of a conventional monitor in the case where the atoms are confined to a region with a large aspect ratio.